This invention relates to electrodes for use in the electrolysis of aqueous alkali metal salt solutions.
Ruthenium dioxide (RuO.sub.2) type electrodes are well known anodes for use in the electrolysis of aqueous alkali metal salt solutions, typically brine electrolysis. A typical example of the ruthenium dioxide type electrodes is disclosed in Japanese Patent Publication No. 46-21884 as comprising a valve metal substrate having a coating of (Ru-Ti)O.sub.2 solid solution applied thereon. Because of its extended lifetime, this electrode has been commercially utilized as a typical metal electrode. Unfortunately, it provides a low anodic current efficiency while evolving a great volume of oxygen.
Japanese Patent Publication No. 50-11330 discloses another electrode having a coating of (Ru-Sn)O.sub.2 solid solution containing at least 50 mol% of SnO.sub.2. This electrode also has an extended lifetime. Contents of RuO.sub.2 of the order of 30 mol% provide sufficient chlorine overvoltage, but at the same time, lead to some disadvantages including low oxygen overvoltage, increased oxygen evolution, and low current efficiency.
In order to increase oxygen overvoltage in these coating compositions, the content of RuO.sub.2 activator should be reduced to about 20 mol%. However, this is unacceptable because chlorine overvoltage is correspondingly increased.
Japanese Patent Publication No. 52-28106 or U.S. Pat. No. 3,776,834 discloses another electrode having a coating of (Ru-Sn-Ti)O.sub.2 solid solution composed of 14 to 20 mol% of RuO.sub.2, 67 to 71 mol% of TiO.sub.2, and 9 to 19 mol% of SnO.sub.2. The coating composition is proposed in order to improve the oxygen overvoltage and hence, the current efficiency of the (Ru-Ti)O.sub.2 solid solution coated electrode. The electrode is described as successful in reducing oxygen evolution by about 20% as compared with the previous (Ru-Ti)O.sub.2 coated electrode. It is still unsatisfactory with respect to oxygen evolution and current efficiency.